The invention relates to a process for preparing carboxylic esters, in particular ethyl formate, by transesterification.
Low molecular weight esters such as formic esters are employed, for example, as fragrances, insecticides, fungicides or in organic synthesis. Processes for preparing low molecular weight esters have been widely described in the literature. An inexpensive possibility is esterification of carboxylic acid and alcohol with subsequent distillation of the ester. In many cases, this process can be carried out very simply in industry because the product in the form of the ester is the lowest-boiling compound.
U.S. Pat. No. 5,302,747 describes a process in which an inert gas is passed through an esterification mixture which comprises an alcohol and a carboxylic acid and is maintained at at least the boiling point of the alcohol in order to drive off the ester.
The preparation of highly pure esters, in particular formic esters, having a purity of greater than 99.5% by weight, in particular greater than 99.8% by weight, is difficult, as will be explained below for the example of the esterification of formic acid with ethanol. The esterification of formic acid with ethanol forms water and ethyl formate. In the distillation of the reaction product, it is possible to separate neither ethanol nor water completely from the ester since both materials form azeotropes with the ester over wide pressure ranges. As a result, highly pure ethyl formate cannot be obtained by this route.
JP 10175916 describes the preparation of highly pure formic esters. The esterification of formic acid and alcohol is carried out by reactive distillation, with the distillate obtained being dewatered by means of acetic anhydride. Although water can be removed by use of desiccants in this process, unreacted alcohol cannot be removed in a comparable way.
WO 2007/099071 describes the preparation of esters by reactive distillation. A carboxylic acid, an alcohol and an entrainer are introduced into a reaction column. The bottom stream comprises the ester formed and unreacted carboxylic acid. The overhead stream comprises unreacted alcohol, water and entrainer.
The preparation of an ester from the carboxylic acid and alcohol has the disadvantage that the acid is generally corrosive and acid-resistant materials have to be used for handling it.